1. Field of the Invention
The present invention relates to an improved spin coating device for a substrate, and more particularly, to a spin coating device for a substrate designed to apply an application liquid onto a substrate rotated at high speed and to form a uniform thin film.
2. Description of the Prior Art
A rotary applicator of such a type as shown in FIG. 2, for example, has been generally known as a spin coating device in which a thin film of a uniform thickness is formed on a substrate by applying an application liquid of relatively low or medium viscosity on the substrate, while the substrate such as a semiconductor substrate is rotated at high speed.
A turntable 2 is used to hold and rotate a substrate above this turntable 2, an application liquid discharge nozzle 4 is arranged. This application liquid discharge nozzle 4 serves to supply an application liquid 3 to a rotating substrate 1.
A splash preventing cup 6 is provided in order to prevent the application liquid from splashing. A splash preventing cover 7 is provided above the substrate 1 held by the turntable 2. This splash preventing cover 7 is employed to prevent splashing of the application liquid 3.
A support mechanism 5 supports the splash preventing cover 7. An exhaust hole 8 is provided towards the exterior from the substrate 1 held by the turntable 2. This exhaust hole 8 is connected by a pipe to a ventilating fan 9 which serves as an external device for exhausting air from the interior of the splash preventing cup 6.
An exhaust hole 12 is connected to an external, waste liquid tank 13. This exhaust hole 12 is employed for exhausting an excess of application liquid due to splashing or the like.
The conventional spin coating device as mentioned above is exemplified by the three kinds of structures described below.
A first type of spin coating device is designed in such a manner that the flow B of external air enters only through a gap 11 between the splash preventing cup 6 and the splash preventing cover 7. The air is supplied to a portion in the vicinity of the peripheral edge of the substrate 1, and a downward air current is introduced toward the exhaust hole 8 by the ventilating fan 9.
In a second type of spin coating device, the gap 11 is sealed, so that external air does not enter and is not expelled through the exhaust hole 8. The spin coating device of this type is so designed that the splash preventing cover 7 is opened upon high speed rotation of the substrate or thereafter.
In a third type of-spin coating device, the splash preventing cover 7 is not provided.
Although drying of the application liquid 3 is usually facilitated by external air to form a thick film, in the case of the first and second types of spin coating devices, it is difficult to supply external air to the entire surface area of the substrate other than portions in the vicinity of the peripheral edge of the substrate. Therefore, drying of the application liquid 3 in sections where it is difficult to supply external air is delayed, with the result that the viscosity of the application liquid 3 is degraded and a thin film is undesirably formed.
As opposed to the two types of spin coating devices described above, the third type of spin coating device is not provided with the splash preventing cover 7 so that external air is sufficiently supplied and a film of satisfactory thickness can be formed.
However, the three types of the conventional spin coating devices have the following problems.
The spin coating device of the first type is so constructed that the flow B of external air enters from the gap 11 between the splash preventing cup 6 and the splash preventing cover 7. Therefore, since a large quantity of external air is supplied to portions in the vicinity of the peripheral edges of the substrate 1, an increase in the thickness of the film is liable to be generated in these portions.
On the other hand, in the case of the spin coating devices of the second and third types, since the relative speed of the air current at the outer periphery is higher, an increase in the thickness of the film is apt to occur at the outer periphery of the substrate. As a result, the thickness of the film formed on the substrate is uneven or an air flow C is generated above the substrate 1 and droplets of misty application liquid 3 are attached on the surface of the substrate 1, easily leading to defects in the quality of the substrate. Further, the flow C is difficult to adjust.